TabPanel including a Container with GridPanel and FormPanel - "this.body is null"

hi everybody,

got a TabPanel with two Tabs - both including a Container with different Panel-Setups.
The Problem is the 2nd Tab, which container includes one GridPanel and one FormPanel.
The FormPanel will be rendered into/over the GridPanel instead next to it and there are just the buttons from the Form visibile.

I suggest that there is something wrong with the layout-management of the Containers/Panels but i have no idea where exactly. my debug console says, that "this.el is null" - if i interpret it right, it crashs in the initEl-function of FormPanel.

if i remove the FormPanel from its Container everything seems to be ok.